YAG laser welding has characteristics mentioned below, and the welding is predominantly used in robot welding of steel materials used for automobiles, etc.
1. The welding can provide a focused laser, whereby high speed welding with low distortion can be performed.
2. Laser absorptance of the YAG laser welding is several times of that of CO2 laser welding, whereby efficient welding can be performed in the YAG laser welding. Moreover, the wavelength in the YAG laser is a tenth of that in CO2 laser welding, whereby the YAG laser welding is not susceptible to an influence by plasma generated in the welding.
3. The laser beam can be transmitted by a flexible optical fiber in the YAG laser welding, whereby handling is easy and an articulated robot can be used. Moreover, transmittance to a point as far as 100 m away can be performed.
4. Time sharing and power sharing can be exhibited by the laser, whereby high usability can be obtained using separated units together in multiple processing stations.
However, when YAG laser welding is applied to laser welding processes, joint accuracy is necessary in many cases, whereby the process is restricted to processing of thin sheet layers in comparison with other types of welding, for example, gas welding, electric welding and arc welding. As a technique to overcome the restriction, welding methods using both a laser and an arc have been conceived and disclosed.
For example, in Japanese Published Unexamined Application No. 53-137044, Japanese Published Examined Application No. 56-49195, Japanese Published Unexamined Application No. 61-232079, Japanese Published Examined Application No. 4-51271, and Japanese Published Unexamined Application No. 5-69165, welding methods, in which welding is performed by using both a TIG arc and a laser, and base materials are melted by using the TIG arc and a laser is irradiated on the melted pool, whereby penetration depth is increased, is disclosed.
In Japanese Published Unexamined Application No. 53-137044, Japanese Published Unexamined Application No. 61-232079, and Japanese Published Unexamined Application No. 7-246484, welding methods in which welding is performed using both a plasma arc and a laser, and base materials are melted by using the plasma arc and the laser is irradiated on the melted pool, whereby penetration depth is increased, is disclosed.
In Japanese Published Unexamined Application No. 10-216972, a combined welding method, in which welding is performed with a butt welding by combining a laser and a consumable electrode type arc, that is, a combined welding method using a laser which is used first and a consumable electrode type arc which is used second, is disclosed.
In Japanese Published Unexamined Application No. 2000-233287, a welding process device in which an optical axis in a laser system and a nozzle electrode is contained and a laser and an arc are used, is disclosed. In the Application, the nozzle electrode is at a coaxial position with the optical axis, and is set at a position against a work piece. Moreover, a high voltage for arc discharge is fed to the nozzle electrode. Furthermore, a laser is irradiated, whereby the work is melted, resulting in performing welding and processing by arc discharge while maintaining the melted condition.
On the other hand, a welding process technique in which both a laser and a filler wire are used is applied to margin expansion to fluctuation, etc., of a groove gap, suppression of weld bead width in high speed welding, and improvement of under-cutting in the butt portion. As the filler wires used in the technique, a filler wire in which current heating is not performed, a so-called “cold wire” or a filler wire in which current heating is performed, and a so-called “hot wire”, are disclosed.
For example, in Japanese Published Unexamined Application No. 4-344873, a hot wire TIG welding, in which power supply is performed from a DC power source to a wire having a diameter of 0.4 mm, wire feed is initiated, contact of wire edge to base materials are detected, and after that, TIG welding is started (p. 4, fifth section, 1.9 and 10), the wire is set in a semi-solid condition by the above-mentioned current heating of the wire from a contact tip, and the wire is fed to a weld pool formed by TIG arc welding, is disclosed. In the Japanese Published Unexamined Application No. 4-344873, it is described that a current heating is performed to an added wire having a diameter of not more than 0.6 mm to form a deposited metal, the added wire is depressed to an edge portion of the contact tip with a wire contacting member through an elastic body, thereby performing the current heating of the wire, plasma arc, electron beam or laser, and in addition, a TIG arc can be used in the melting of the base materials.
The welding in which only a YAG laser is used has the problems mentioned below.
1. In the YAG laser, welding is performed by focusing a light beam, thereby increasing energy density. However, the spot diameter in the focused laser is 0.3 to 1 mm.
Therefore, in the case of the butt welding, when a butt gap (for example, FIG. 3C) exists, the laser goes through the gap, and welding defects are generated. Accordingly, the cutting of edge in a weld zone in the welded material 41, 42 and method for holding the weld zone is rigidly requested.
Moreover, in the case of a lap welding, a joint boundary face width of a penetration bead from an upper plate 44 to a lower plate 43 (for example, FIG. 3D) is narrow, whereby excellent joint strength cannot be obtained.
Furthermore, also in the case of a fillet welding of members 45 and 46, the penetration width (for example, FIG. 3E) cannot be adequately obtained similar to the lap welding, whereby excellent joint strength cannot be obtained.
Additionally, when a welding is performed by simultaneously feeding filler wire to a YAG laser weld zone for the purpose of improving reinforcement of the weld and surface bead, energy is consumed in melting of the filler wire, whereby weldability is decreased.
2. Initial cost is high for the YAG laser device, whereby equipment cost is extremely high in the case of introducing multiple units for an installation, for example, in an automobile production line.
In Japanese Published Unexamined Application (utility model) No. 62-82194, a nozzle structure mentioned below is disclosed. That is, a laser beam is absorbed by a plasma-activated gas and a metal vapor, that is a plume, in a keyhole generated in base materials for welding by laser irradiation or above the keyhole in the laser welding, whereby it is regarded that a welding in which the penetration is deep cannot be performed, the plume is guided to an exhaust path formed outside of a laser path by blowing a shielding gas into the keyhole, and an aspiration and exhaust are performed. The structure aims to effectively increase laser heat input for the base materials. In the YAG laser wavelength, a YAG laser is rarely absorbed by a plasma-activated gas and a metal vapor, that is a plume, in the keyhole or above the keyhole. When exhaust is performed by absorbing a YAG laser by a plasma-activated gas and a metal vapor, strength for holding the keyhole down disappears, whereby the depth of the penetration becomes shallow.
In the Japanese Published Unexamined Application No. 53-137044, the Japanese Published Examined Application No. 56-49195, the Japanese Published Unexamined Application No. 61-232079, the Japanese Published Examined Application No. 4-51271, and the Japanese Published Unexamined Application No. 5-69165, a combined welding by a TIG and a laser in which base materials are melted by using a TIG arc to obtain a weld pool of the base materials and a laser is irradiated on the weld pool, whereby the penetration depth is increased, is disclosed. The disclosure in the documents aims to form a shallow and a broad weld pool by the TIG and to obtain a narrow and deep penetration by the laser. Melted width of the surface of the base materials is increased by the TIG and by a broad weld pool, for example, the butt gap shown in FIG. 3C is filled up, deep penetration is formed by the laser, for example, the penetration width shown in FIG. 3E is broadened, that is, penetration is deepened, whereby weldability and weld quality are increased in comparison with using the laser alone.
However, only the laser penetrates to deep locations, whereby penetration width is narrow and welding strength is low. For example, joint boundary face width in the lap welding shown in FIG. 3D is narrow, whereby increasing of the width is desired. The demand exists similarly for the plasma arc in the Japanese Published Unexamined Application No. 7-246484 and the Japanese Published Unexamined Application No. 10-216972, and in the Japanese Published Unexamined Application No. 2000-233287.
As a welding method in which a laser and a filler wire are both used, conventionally, a filler wire in which current heating is not performed, that is, a so-called “cold wire” is generally used, a part of the laser energy for melting the base materials are consumed by the melting of the filler wire, whereby there is a problem in that weldability is decreased. On the other hand, in the Japanese Publication Unexamined Application No. 4-344873, hot wire TIG using a filler wire having a diameter of not more than 0.6 mm with a purpose of forming a fusil and stable welding bead in high speed welding of a thin plate is disclosed. Moreover, in the document, changing the TIG arc to a laser irradiation is proposed.
However, sufficient penetration depth cannot be obtained, whereby welding strength is low. Specifically, the technique in the proposal is unsuitable for the lap welding and fillet welding in which deep penetration depth is desired.